Process and Apparatus for Nitrogen Production

ABSTRACT

A process and apparatus for producing nitrogen by which product nitrogen can be efficiently supplied. The nitrogen producing process comprises: a first separation step in which feed air is distilled at a low temperature; a first indirect heat exchange step in which a first nitrogen gas and a first oxygen-enriched liquefied fluid which have been separated in the first separation step are subjected to indirect heat exchange to obtain a first liquefied nitrogen and a first oxygen-enriched gas fluid; a second separation step in which the first oxygen-enriched gas fluid is distilled at a low temperature; a second indirect heat exchange step in which a second nitrogen gas and a second oxygen-enriched liquefied fluid which have been separated in the second separation step are subjected to indirect heat exchange to obtain a second liquefied nitrogen and a second oxygen-enriched gas fluid; a cold generating step in which the second oxygen-enriched gas fluid is adiabatically expanded to generate cold; a first product recovery step in which part of the first nitrogen gas is discharged as a first product nitrogen gas; and a second product recovery step in which part of the second nitrogen gas is discharged as a second product nitrogen gas after the cold is recovered.

TECHNICAL FIELD

The present invention relates to a nitrogen producing process and apparatus, and, specifically to a process and apparatus which collect nitrogen by performing separation refinement on feed air by cryogenic liquefaction separation, and, more particularly, to an optimal nitrogen producing process and apparatus for collecting product nitrogen with a pressure range of about 0.6 to 1.1 MPa (absolute pressure; hereinafter the same).

BACKGROUND ART

Many industrial productions of nitrogen employ air liquefaction separation by cryogenic liquefaction separation, and various processes and apparatuses have been proposed to reduce the power consumption per product nitrogen gas and increase the range of quantity reduction. For example, a nitrogen producing process and apparatus have been proposed which reduce the power consumption per product nitrogen and increase the range of quantity reduction of product nitrogen by using a first rectification column and a second rectification column having different operational pressures (e.g., see Patent Document 1).

Patent Document 1: Japanese Unexamined Patent Publication No. 2003-156284 DISCLOSURE OF THE INVENTION Problem to be Solved by the Invention

However, a further reduction of the power consumption per product nitrogen is demanded. Particularly, because there is a large demand for product nitrogen with a pressure of about 0.6 to 1.1 MPa, developments on a process and apparatus capable of efficiently producing product nitrogen are desired. For example, a double-column nitrogen producing apparatus described in the aforementioned Japanese Unexamined Patent Publication No. 2003-156284 generates cold by feeding a part of a first oxygen-enriched gas fluid withdrawn from a first rectification column and vaporized by a first condenser, so that the process quantity of a second rectification column is small. In the case of a small-sized nitrogen producing apparatus, there is some room for improvement, such as the narrow range of selection of the types of expansion turbines due to the small process quantity of the expansion turbine.

It is therefore an object of the invention to provide a nitrogen producing process and a nitrogen producing apparatus which can efficiently and economically supply product nitrogen with a pressure range of about 0.6 to 1.1 MPa using a dual column nitrogen producing apparatus, and can easily select an optimal constituting device.

Means for Solving the Problem

A first aspect of a nitrogen producing process for collecting product nitrogen by performing cryogenic liquefaction separation of feed air according to the present invention includes a first separation step of performing low temperature distillation of compressed, purified and cooled feed air at a pressure of 0.8 MPa or higher and 1.1 MPa or lower to separate the feed air into a first nitrogen gas and a first oxygen-enriched liquefied fluid; a first indirect heat exchange step of performing an indirect heat exchange between the first nitrogen gas and the first oxygen-enriched liquefied fluid to condense the first nitrogen gas to obtain a first liquefied nitrogen and, at the same time, vaporize the first oxygen-enriched liquefied fluid to obtain a first oxygen-enriched gas fluid; a second separation step of performing low temperature distillation of the first oxygen-enriched gas fluid at a pressure of 0.4 MPa or higher and lower than the pressure in the first separation step to separate the first oxygen-enriched gas fluid into a second nitrogen gas and a second oxygen-enriched liquefied fluid; a second indirect heat exchange step of performing an indirect heat exchange between the second nitrogen gas and the second oxygen-enriched liquefied fluid to condense the second nitrogen gas to obtain a second liquefied nitrogen and, at the same time, vaporize the second oxygen-enriched liquefied fluid to obtain a second oxygen-enriched gas fluid; a cold generating step of generating cold needed for the operation by performing adiabatic expansion of the second oxygen-enriched gas fluid; a first product collecting step of discharging a part of the first nitrogen gas as a first product nitrogen gas after the cold is recovered; and a second product collecting step of discharging a part of the second nitrogen gas as a second product nitrogen gas after cold recovery.

A second aspect of a nitrogen producing process of the present invention includes a first separation step of performing low temperature distillation of compressed, purified and cooled feed air at a pressure of 0.6 MPa or higher and 1.1 MPa or lower to separate the feed air into a first nitrogen gas and a first oxygen-enriched liquefied fluid; a first indirect heat exchange step of performing an indirect heat exchange between the first nitrogen gas and the first oxygen-enriched liquefied fluid to condense the first nitrogen gas to obtain a first liquefied nitrogen and, at the same time, vaporize the first oxygen-enriched liquefied fluid to obtain a first oxygen-enriched gas fluid; a second separation step of performing low temperature distillation of the first oxygen-enriched gas fluid at a pressure of 0.3 MPa or higher and lower than the pressure in the first separation step to separate the first oxygen-enriched gas fluid into a second nitrogen gas and a second oxygen-enriched liquefied fluid; a second indirect heat exchange step of performing an indirect heat exchange between the second nitrogen gas and the second oxygen-enriched liquefied fluid to condense the second nitrogen gas to obtain a second liquefied nitrogen and, at the same time, vaporize the second oxygen-enriched liquefied fluid to obtain a second oxygen-enriched gas fluid; a cold generating step of generating cold needed for the operation by performing adiabatic expansion of a part of the feed air; an air feeding step of introducing the feed air undergone the cold generating step to an intermediate stage of the second separation step; a first product collecting step of discharging a part of the first nitrogen gas as a first product nitrogen gas after the cold is recovered; and a second product collecting step of discharging a part of the second nitrogen gas as a second product nitrogen gas after cold recovery. The first and second aspects of the nitrogen producing process may include a step of compressing the second product nitrogen gas.

A first aspect of a nitrogen producing apparatus for collecting product nitrogen by performing cryogenic liquefaction separation of feed air according to the present invention is a nitrogen producing apparatus for collecting product nitrogen by performing cryogenic liquefaction separation of feed air, the nitrogen producing apparatus including a first rectification column which performs low temperature distillation of compressed, purified and cooled feed air at a pressure of 0.8 MPa or higher and 1.1 MPa or lower to separate the feed air into a first nitrogen gas at the upper portion of the column and a first oxygen-enriched liquefied fluid at the bottom portion of the column; a first condenser which performs an indirect heat exchange between the first nitrogen gas and the first oxygen-enriched liquefied fluid to condense the first nitrogen gas to obtain a first liquefied nitrogen and, at the same time, vaporize the first oxygen-enriched liquefied fluid to obtain a first oxygen-enriched gas fluid; a second rectification column which performs low temperature distillation of the first oxygen-enriched gas fluid at a pressure of 0.4 MPa or higher and lower than the pressure of the first rectification column to rectify and separate the first oxygen-enriched gas fluid into a second nitrogen gas at the upper portion of the column and a second oxygen-enriched liquefied fluid at the bottom portion of the column; a second condenser which performs an indirect heat exchange between the second nitrogen gas and the second oxygen-enriched liquefied fluid to condense the second nitrogen gas to obtain a second liquefied nitrogen and, at the same time, vaporize the second oxygen-enriched liquefied fluid to obtain a second oxygen-enriched gas fluid; an expansion turbine which generates cold needed for the operation of the apparatus by performing adiabatic expansion of the second oxygen-enriched gas fluid; a first product recovery passage for discharging a part of the first nitrogen gas as a first product nitrogen gas after the cold is recovered; and a second product recovery passage for discharging a part of the second nitrogen gas as a second product nitrogen gas after cold recovery.

A second aspect of a nitrogen producing apparatus of the invention includes a first rectification column which performs low temperature distillation of compressed, purified and cooled feed air at a pressure of 0.6 MPa or higher and 1.1 MPa or lower to separate the feed air into a first nitrogen gas at the upper portion of the column and a first oxygen-enriched liquefied fluid at the bottom portion of the column; a first condenser which performs an indirect heat exchange between the first nitrogen gas and the first oxygen-enriched liquefied fluid to condense the first nitrogen gas to obtain a first liquefied nitrogen and, at the same time, vaporize the first oxygen-enriched liquefied fluid to obtain a first oxygen-enriched gas fluid; a second rectification column which performs low temperature distillation of the first oxygen-enriched gas fluid at a pressure of 0.3 MPa or higher and lower than the pressure of the first rectification column to rectify and separate the first oxygen-enriched gas fluid into a second nitrogen gas at the upper portion of the column and a second oxygen-enriched liquefied fluid at the bottom portion of the column; a second condenser which performs an indirect heat exchange between the second nitrogen gas and the second oxygen-enriched liquefied fluid to condense the second nitrogen gas to obtain a second liquefied nitrogen and, at the same time, vaporize the second oxygen-enriched liquefied fluid to obtain a second oxygen-enriched gas fluid; an expansion turbine which generates cold needed for the operation of the apparatus by performing adiabatic expansion of a part of the feed air; an air feeding passage for introducing the feed air passed through the expansion turbine to an intermediate stage of the second rectification column; a first product recovery passage for discharging a part of the first nitrogen gas as a first product nitrogen gas after the cold is recovered; and a second product recovery passage for discharging a part of the second nitrogen gas as a second product nitrogen gas after cold recovery.

The first and second aspects of the nitrogen producing apparatus can include a nitrogen compressor which compresses the second product nitrogen gas. The second rectification column can have a liquefied nitrogen feeding passage for introducing liquefied nitrogen from outside the apparatus.

EFFECT OF THE INVENTION

According to the first aspect of the nitrogen producing process and nitrogen producing apparatus according to the present invention, the second oxygen-enriched gas fluid vaporized by the second condenser which performs the second indirect heat exchange step is used as a fluid to be introduced into the expansion turbine which performs the cold generating step. Therefore, it is unnecessary to branch the first oxygen-enriched gas fluid to the expansion turbine and almost the entire first oxygen-enriched gas fluid is introduced into the second rectification column, so that the process quantity of the second rectification column becomes greater than the conventional quantity, thus making it possible to increase the quantity of product nitrogen to be collected from the second rectification column.

According to the second aspect of the nitrogen producing process and nitrogen producing apparatus according to the present invention, a part of compressed, purified and cooled feed air is branched and introduced into the expansion turbine, which performs the cold generating step, to be adiabatically expanded, thereby generating cold needed for the operation of the apparatus, and is then introduced into the intermediate stage of the second rectification column. Therefore, it is possible to efficiently obtain a necessary quantity of cold and make the process quantity of the second rectification column greater than the conventional quantity, so that the quantity of product nitrogen to be collected from the second rectification column can be increased.

Further, the provision of the nitrogen compressor to compress the second product nitrogen gas can make the pressure of the second product nitrogen gas the same as the pressure of the first product nitrogen gas and supply the second product nitrogen gas to where it is used. The introducing of the liquefied nitrogen from outside the apparatus makes it unnecessary to feed a part of the first oxygen-enriched liquefied fluid for supplementing cold to the second rectification column, and, further can reduce the process quantity of the expansion turbine.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a systematic diagram of a nitrogen producing apparatus showing a first embodiment of the present invention (low-pressure turbine process).

FIG. 2 is a systematic diagram of a nitrogen producing apparatus showing a second embodiment of the present invention (air turbine process).

FIG. 3 is a systematic diagram of a nitrogen producing apparatus relating to a conventional intermediate-pressure turbine process used in comparison with the present invention.

FIG. 4 is a systematic diagram showing one example of a nitrogen producing apparatus relating to a conventional low-pressure process using one rectification column.

FIG. 5 is a diagram showing a power consumption per production for each of product pressures in the low-pressure process, low-pressure turbine process and air turbine process.

FIG. 6 is a diagram showing ranges of a product pressure and product flow rate in which the low-pressure turbine process and the air turbine process are advantageous.

DESCRIPTION OF REFERENCE NUMERALS

-   -   11 . . . first rectification column, 12 . . . first condenser,         13 . . . second rectification column, 14 . . . second condenser,         15 . . . low-pressure expansion turbine, 16 . . . main heat         exchanger, 17 . . . depressurizing valve, 18 . . . nitrogen         compressor, 19 . . . depressurizing valve, 20 . . .         depressurizing valve, 32 . . . feed air inlet passage, 35 . . .         first product recovery passage, 43 . . . second product recovery         passage, 72 . . . air turbine, 73 . . . air feeding passage, 74         . . . oxygen-enriched liquefied fluid feeding passage, 85 . . .         medium-pressure expansion turbine

BEST MODE FOR CARRYING OUT THE INVENTION

FIG. 1 shows the first embodiment of the present invention, and is a systematic diagram of a nitrogen producing apparatus to which the first aspect of the nitrogen producing process and nitrogen producing apparatus is applied.

The nitrogen producing apparatus illustrated by the embodiment includes a first rectification column 11 which performs low temperature distillation of compressed, purified and cooled feed air at a pressure of 0.8 MPa or higher and 1.1 MPa or lower to separate the feed air into a first nitrogen gas at the upper portion of the column and a first oxygen-enriched liquefied fluid at the bottom portion of the column, a first condenser 12 which performs an indirect heat exchange between the first nitrogen gas and the first oxygen-enriched liquefied fluid to condense the first nitrogen gas to obtain a first liquefied nitrogen and, at the same time, vaporize the first oxygen-enriched liquefied fluid to obtain a first oxygen-enriched gas fluid, a second rectification column 13 which performs low temperature distillation of the first oxygen-enriched gas fluid at a pressure of 0.4 MPa or higher and lower than the pressure of the first rectification column 11 to rectify and separate the first oxygen-enriched gas fluid into a second nitrogen gas at the upper portion of the column and a second oxygen-enriched liquefied fluid at the bottom portion of the column, a second condenser 14 which performs an indirect heat exchange between the second nitrogen gas and the second oxygen-enriched liquefied fluid to condense the second nitrogen gas to obtain a second liquefied nitrogen and, at the same time, vaporize the second oxygen-enriched liquefied fluid to obtain a second oxygen-enriched gas fluid, and an expansion turbine (hereinafter called “low-pressure expansion turbine”) 15 which generates cold needed for the operation of the apparatus by performing adiabatic expansion of the second oxygen-enriched gas fluid.

Compressed, purified feed air flows into a main heat exchanger 16 from a passage 31, where it is cooled to a predetermined temperature by heat exchange with a product nitrogen gas and waste gas. The cooled feed air is introduced into the lower portion of the first rectification column 11 through a feed air inlet passage 32, and is separated into a nitrogen gas at the upper portion of the column (first nitrogen gas) and an oxygen-enriched liquefied fluid at the bottom portion of the column (first oxygen-enriched liquefied fluid) (first separation step). A part of the first nitrogen gas withdrawn from the column's top portion to a passage 33 is branched into a passage 34 and is warmed by heat exchange with the feed air in the main heat exchanger 16, is subjected to cold recovery, and is then derived as a first product nitrogen gas through a first product recovery passage 35 (first product recovery step). The remaining first nitrogen gas is introduced into the first condenser 12 through a passage 36.

The first oxygen-enriched liquefied fluid is withdrawn from the lower portion of the first rectification column 11 to a passage 37, is depressurized by a depressurizing valve 17 to a pressure which provides a temperature at which the first nitrogen gas can be liquefied, and is introduced into the first condenser 12 through a passage 38. The first oxygen-enriched liquefied fluid and the first nitrogen gas are subjected to indirect heat exchange in the first condenser 12, so that the first nitrogen gas is condensed to be liquefied nitrogen (first liquefied nitrogen) and, at the same time, the first oxygen-enriched liquefied fluid is vaporized to be an oxygen-enriched gas fluid (first oxygen-enriched gas fluid) (first indirect heat exchange step). The first liquefied nitrogen is introduced into the upper portion of the first rectification column 11 through a passage 39 as a reflex liquid.

The first oxygen-enriched gas fluid vaporized in the first condenser 12 is introduced into the lower portion of the second rectification column 13 through a passage 40, and is separated to a nitrogen gas at the upper portion of the column (second nitrogen gas) and an oxygen-enriched liquefied fluid at the bottom portion of the column (second oxygen-enriched liquefied fluid) by low temperature distillation in the second rectification column 13 (second separation step). A part of the second nitrogen gas withdrawn from the column's top portion to a passage 41 is branched into a passage 42 and is warmed by heat exchange with the feed air in the main heat exchanger 16, is subjected to cold recovery, is then derived as a second product nitrogen gas from a second product recovery passage 43 (second product recovery step), is compressed to a predetermined pressure in a nitrogen compressor 18, and is sent through a passage 44 to where it is used (compression step). The remaining second nitrogen gas is introduced into the second condenser 14 through a passage 45.

The second oxygen-enriched liquefied fluid is withdrawn from the lower portion of the second rectification column 13 to a passage 46, is combined with the first oxygen enriched liquefied fluid which is branched into a passage 47 from the passage 37 and is depressurized by a depressurizing valve 19 to the pressure of the second oxygen-enriched liquefied fluid, is then depressurized by a depressurizing valve 20 to a pressure which provides a temperature at which the second nitrogen gas can be liquefied, and is introduced into the second condenser 14 through a passage 48. In the second condenser 14, a combined fluid with the first oxygen-enriched liquefied fluid and the second oxygen-enriched liquefied fluid is subjected to indirect heat exchange with the second nitrogen gas, so that the second nitrogen gas is condensed to be liquefied nitrogen (second liquefied nitrogen) and, at the same time, the combined fluid is vaporized to be an oxygen-enriched gas fluid (second oxygen-enriched gas fluid) (second indirect heat exchange step). The second liquefied nitrogen is introduced into the upper portion of the second rectification column 13 through a passage 49 as a reflex liquid.

The second oxygen-enriched gas fluid output to a passage 50 from the second condenser 14 is branched into a passage 51 and a passage 52. Most of the second oxygen-enriched gas fluid is introduced into the main heat exchanger 16 through the passage 52, is warmed to an intermediate temperature, and is withdrawn to a passage 53, and is introduced into the low-pressure expansion turbine 15. The remaining second oxygen-enriched gas fluid branched into the passage 51 is depressurized by a valve 21. The second oxygen-enriched gas fluid which has generated cold needed for the operation of the apparatus by adiabatic expansion in the low-pressure expansion turbine 15 (cold generation step) passes through a passage 54, is combined with the second oxygen-enriched gas fluid which is branched into the passage 51 and is depressurized by the valve 21, is subjected to cold recovery in the main heat exchanger 16, and is then output as a waste gas through a passage 55. A part of the waste gas is used for the regeneration of an adsorber which purifies feed air.

A small quantity of the first oxygen-enriched liquefied fluid branched into the passage 47 is used for cold supplement of the second rectification column 13. Most of the first oxygen-enriched liquefied fluid is introduced into the first condenser 12. The first oxygen-enriched liquefied fluid branched into the passage 47 may be introduced into the intermediate stage of the second rectification column 13. There is a case where a part of the first oxygen-enriched gas fluid flowing through the passage 40 is bypassed to the passage 50 via a regulating valve to control the pressure of the second rectification column 13. In this case too, a small quantity of the first oxygen-enriched gas fluid is bypassed to the passage 50 from the passage 40, and most of the first oxygen-enriched gas fluid is introduced into the second rectification column 13. Therefore, all or most of the first oxygen-enriched liquefied fluid separated at the first rectification column 11 is vaporized to the first oxygen-enriched gas fluid in the first condenser 12. Thus, all or most of the first oxygen-enriched gas fluid is introduced into the second rectification column 13.

Normally, the second product nitrogen gas is compressed by the nitrogen compressor 18 to same pressure of the first product nitrogen gas which is output through the first product recovery passage 35, but an arbitrary pressure can be selected according to the situation of where it is used. There is a case where the second product nitrogen gas is output through the second product recovery passage 43 without compression by the nitrogen compressor 18. It is possible to provide a compressor which compresses the first product nitrogen gas, as needed.

All of the second oxygen-enriched gas fluid passing through the passage 50 can be introduced into the low-pressure expansion turbine 15, and so a part of the first liquefied nitrogen in passage 39 and/or the second liquefied nitrogen in passage 49 can be collected as product liquefied nitrogen using the cold increased by the increased process quantity.

The minimum operation pressures of both rectification columns 11, 13 are determined by the pressure of the waste gas withdrawn through the passage 55. That is, since the second oxygen-enriched gas fluid (waste gas) derived from the low-pressure expansion turbine 15 is subjected to cold recovery in the main heat exchanger 16 and is then used for regeneration of an adsorber, the second oxygen-enriched gas fluid in the passage 54 at the outlet portion of the low-pressure expansion turbine 15 must have a pressure that the fluid can be released to atmosphere against pressure drops of equipments (main heat exchanger 16 and so on) and regeneration of the adsorber.

Further, in order to generate cold needed for the operation of the apparatus by the low-pressure expansion turbine 15, the turbine must have a predetermined expansion ratio. So, the pressure of the second oxygen-enriched gas fluid in the passage 53 at the inlet portion of the low-pressure expansion turbine 15 must be 0.16 MPa or higher.

In the second condenser 14, since the second oxygen-enriched liquefied fluid and the second nitrogen gas are indirectly heat-exchanged with each other, the second nitrogen gas is liquefied and the second oxygen-enriched liquefied fluid is vaporized. Therefore, in the case where the minimum pressure of the second oxygen-enriched gas fluid 50 is about 0.16 MPa, the pressure at the top portion of the second rectification column 13 thereof, which is the pressure of the second nitrogen gas, should be about 0.4 MPa or higher.

Further, in the case where the pressure at the top portion of the second rectification column 13 thereof is about 0.4 MPa or higher, the pressure at the top portion of the first rectification column 11 which is the pressure of the first nitrogen gas should be about 0.8 MPa or higher, because the pressure of the second rectification column 13 is the pressure of the first oxygen-enriched gas fluid to be indirectly heat-exchanged with the first nitrogen gas in the first condenser 12 as mentioned above.

That is, the operation pressure of the first rectification column 11 should be set to 0.8 MPa or higher, the operation pressure of the second rectification column 13 should be set to 0.4 MPa or higher, and lower than the operation pressure of the first rectification column 11 due to the necessity to receive the first oxygen-enriched gas fluid.

FIG. 2 shows the second embodiment of the present invention, and is a systematic diagram of a nitrogen producing apparatus to which the second aspect of the process and apparatus for nitrogen production is applied. In the following description, same reference numerals are given to same constituting elements as those of the illustrated nitrogen producing apparatus of the first embodiment and their detailed descriptions will be omitted.

In the illustrated nitrogen producing apparatus of the embodiment, a feed air compressed and purified in passage 31 flows into the main heat exchanger 16. A part of the air is branched into a passage 71 at an intermediate temperature, is withdrawn from the main heat exchanger 16, is introduced into an expansion turbine (hereinafter called “air turbine”) 72 which uses a part of feed air. The air is expanded adiabatically to generate cold (cold generation step), and is then introduced into the intermediate stage of the second rectification column 13 through an air feeding passage 73 (air feeding step). A part of the first oxygen-enriched liquefied fluid branched into the passage 47 from the passage 37 is depressurized by the depressurizing valve 19, and is introduced into the intermediate stage of the second rectification column 13 as a cold supplement through an oxygen-enriched liquefied fluid feeding passage 74.

Though the ratio of the feed air to be branched into the air turbine 72 can be set adequately according to the necessary quantity of cold and the expansion turbine efficiency, it is normally set to an adequate range of 10 to 20%. The feed positions of the air feeding passage 73 and the oxygen-enriched liquefied fluid feeding passage 74 to the second rectification column 13 can be set arbitrarily according to the design conditions, but are normally set to a same position.

Most of the feed air introduced into the main heat exchanger 16 through the passage 31 is cooled to a predetermined temperature, and is then introduced into the lower portion of the first rectification column 11 through the passage 32. A part of the first nitrogen gas separated by low temperature distillation in the first rectification column 11 is output as the first product nitrogen gas, and the remaining of the first nitrogen gas is indirectly heat-exchanged in the first condenser 12 to obtain a first liquefied nitrogen.

The feed air, which is branched into the passage 71 to be introduced into the air turbine 72 and is output to the air feeding passage 73 after adiabatic expansion in the air turbine 72, is introduced into the intermediate stage of the second rectification column 13 as a rising vapor. The feed air, the first oxygen-enriched gas fluid through the passage 40, the first oxygen-enriched liquefied fluid through the oxygen-enriched liquefied fluid feeding passage 74 and the second liquefied nitrogen through the passage 49 are distilled at a low temperature, and are separated to a nitrogen gas (second nitrogen gas) at the upper column portion and an oxygen-enriched liquefied fluid at the bottom column portion (second oxygen-enriched liquefied fluid). A part of the second nitrogen gas is output as the second product nitrogen gas, passing through the passage 42, the main heat exchanger 16 and the passage 43. The second oxygen-enriched gas fluid, which is vaporized by indirect heat exchange in the second condenser 14 and is output to the passage 50, is recovered its cold in the main heat exchanger 16, and is output as a waste gas through the passage 55.

In the embodiment, the second oxygen-enriched gas fluid (waste gas), which is vaporized in the second condenser 14 and is output to the passage 50, is derived through the main heat exchanger 16 without via the expansion turbine, and is only used for regeneration of an adsorber. In this embodiment, unlike the first embodiment, there is no need to consider the expansion in the expansion turbine, so the output pressure of waste gas from the second condenser 14 can be set to about the atmospheric pressure. To liquefy the second nitrogen gas with the second oxygen-enriched liquefied fluid whose pressure is near the atmospheric pressure, the pressure at the top portion of the second rectification column 13 should be set to approximately 0.3 MPa or higher. Because the pressure of the second rectification column 13 is the same as that of the first oxygen-enriched gas fluid which is indirectly heat-exchanged with the first nitrogen gas in the first condenser 12, as mentioned above, the pressure at the top portion of the first rectification column 11, which is the pressure of the first nitrogen gas, should be set to approximately 0.6 MPa or higher.

In both embodiments, a part of the first oxygen-enriched liquefied fluid withdrawn from the first rectification column 11 is introduced into the passage 46 or the second rectification column to supply cold needed for the operation of the second rectification column 13. In case there are another cold supplying means as a cold source, they can be used. For example, by a supply of liquefied nitrogen from outside to the second rectification column 13, all of the first oxygen-enriched liquefied fluid can be introduced into the second rectification column. Then, a fluid rate to be introduced into the expansion turbine can be reduced, and the product flow rate increases consequently. A kind of a cold fluid (for example, liquefied nitrogen) or a cold source in the outside of the apparatus can be adequately selected according to the operational state of the apparatus or the necessary quantity of cold. Therefore, liquefied nitrogen may be introduced into the first rectification column 11.

Next, the results of comparison between the nitrogen producing apparatuses illustrated in each embodiment mentioned before and a conventional nitrogen producing apparatus will be explained. FIG. 3 shows a systematic diagram of a conventional dual column nitrogen producing apparatus used in the comparison. As the pressure of the fluid to be introduced into the expansion turbine is an intermediate pressure between the pressure of the second oxygen-enriched gas fluid 53 in the first embodiment and the pressure of a part of the feed air 71 illustrated in the second embodiment, hereunder, the first embodiment is called a low-pressure turbine process, the second embodiment is called an air turbine process, and the prior art is called an intermediate-pressure turbine process. In FIG. 3, same reference numerals are given to same constituting elements as those of the illustrated nitrogen producing apparatus of the first embodiment and their detailed descriptions will be omitted.

The low-pressure turbine process of the first embodiment differs from the intermediate-pressure turbine process of the prior art in the fluid that is introduced into the expansion turbine. That is, in the low-pressure turbine process, the first oxygen-enriched gas fluid is not introduced into the low-pressure expansion turbine 15, but most of the second oxygen-enriched gas fluid in the passage 50 from the second condenser 14 is introduced into the turbine 15 through the passage 53; whereas in the intermediate-pressure turbine process, as shown in FIG. 3, a part of the first oxygen-enriched gas fluid vaporized in the first condenser 12 is branched into a passage 81 from the passage 40, a part of the fluid 81 is further branched into a passage 82. Then, the remaining fluid 81 is introduced into the main heat exchanger 16 through a passage 83, is withdrawn to a passage 84 at an intermediate temperature, and is introduced into an expansion turbine (medium-pressure expansion turbine) 85.

Most of the first oxygen-enriched gas fluid of the passage 40 is introduced into the lower portion of the second rectification column 13 through a passage 40 a. Therefore, the quantity of the first oxygen-enriched gas fluid to be introduced into the second rectification column in the intermediate-pressure turbine process is less than that of the low-pressure turbine process, because a part of the fluid 40 is branched into the passage 81.

The first oxygen-enriched gas fluid adiabatically expanded by the medium-pressure expansion turbine 85 and introduced into a passage 86 is combined with the second oxygen-enriched gas fluid reduced in pressure by a depressurizing valve 87 from the passage 50 and the first oxygen-enriched gas fluid reduced in pressure by a depressurizing valve 88 in the passage 82. A cold of the combined fluid is recovered in the main heat exchanger 16, and the fluid is then led as a waste gas from the passage 55.

The flow rates (relative values), pressures and oxygen concentrations of the fluids flowing through main passages A to M are respectively illustrated in Table 1 (first embodiment: low-pressure turbine process), Table 2 (second embodiment: air turbine process), and Table 3 (conventional apparatus: intermediate-pressure turbine process), in cases where the low-pressure turbine process of the first embodiment, the air turbine process of the second embodiment, and the conventional intermediate-pressure turbine process are operated under nearly the same pressure conditions.

With regard to numerals A to M described in the individual tables, as shown in FIGS. 1 to 3, numeral A denotes feed air supplied to the passage 31, numeral B denotes feed air supplied to the first rectification column in the passage 32, numeral C denotes the first product nitrogen gas in the first product recovery passage 35, numeral D denotes the first oxygen-enriched liquefied fluid output from the first rectification column in the passage 37, numeral E denotes the branched first oxygen-enriched liquefied fluid in the passage 47, numeral F denotes the first oxygen-enriched liquefied fluid introduced into the first condenser in the passage 38, numeral G denotes the first oxygen-enriched gas fluid introduced into the second rectification column in the passage 40 or the passage 40 a, numeral H denotes the second product nitrogen gas in the second product recovery passage 43, numeral I denotes the compressed second product nitrogen gas in the passage 44, numeral J denotes the second oxygen-enriched gas fluid output from the second condenser in the passage 50, numeral K denotes a fluid introduced into the expansion turbine, that is, the second oxygen-enriched gas fluid in the passage 53 in the low-pressure turbine process, the feed air in the passage 71 in the air turbine process, and the first oxygen-enriched gas fluid in the passage 84 in the intermediate-pressure turbine process. Numeral L denotes the second oxygen-enriched gas fluid in the passage 51 in the low-pressure turbine process, the feed air in the passage 73 in the air turbine process, and the first oxygen-enriched gas fluid in the passage 82 in the intermediate-pressure turbine process, and numeral M denotes the discharged second oxygen-enriched gas fluid (waste gas) in the passage 55.

TABLE 1 Numeral A B C D E F G Flow rate [-] 100 100 40 60 2 58 58 Pressure [MPa] 0.98 0.97 0.94 0.97 0.97 0.51 0.51 Oxygen 21% 21% 0.1 ppm 35% 35% 35% 35% concentration Numeral H I J K L M Flow rate [-] 22 22 38 31 7 38 Pressure [MPa] 0.49 0.94 0.19 0.18 0.13 0.12 Oxygen 0.1 ppm 0.1 ppm 55% 55% 55% 55% concentration

TABLE 2 Numeral A B C D E F G Flow rate [-] 100 85 33 52 5 47 47 Pressure [MPa] 0.98 0.97 0.94 0.97 0.97 0.51 0.51 Oxygen 21% 21% 0.1 ppm 35% 35% 35% 35% concentration Numeral H I J K L M Flow rate [-] 30 30 37 15 15 37 Pressure [MPa] 0.49 0.94 0.18 0.97 0.51 0.17 Oxygen 0.1 ppm 0.1 ppm 57% 21% 21% 57% concentration

TABLE 3 Numeral A B C D E F G Flow rate [-] 100 100 40 60 2 58 50 Pressure [MPa] 0.98 0.97 0.94 0.97 0.97 0.51 0.51 Oxygen 21% 21% 0.1 ppm 35% 35% 35% 35% concentration Numeral H I J K L M Flow rate [-] 19 19 33 7 1 41 Pressure [MPa] 0.49 0.94 0.19 0.50 0.13 0.12 Oxygen 0.1 ppm 0.1 ppm 55% 35% 35% 51% concentration

First, in Table 1 and Table 3, because the pressures of the first rectification column 11 in both tables are the same, the yields of nitrogen become the same, that is, the flow rates of the first product nitrogen gas (C) with respect to the flow rate of 100 of the feed air (B) are 40 in both tables. Likewise, the flow rates of the first oxygen-enriched liquefied fluid (D) output from the first rectification column 11 are both 60.

In the intermediate-pressure turbine process, however, because a part of the first oxygen-enriched gas fluid from the first condenser 12 in the passage 40 is branched for the medium-pressure expansion turbine 85, the first oxygen-enriched gas fluid (G) to be introduced into the second rectification column 13 is reduced by the flow rate (7+1=8) of the first oxygen-enriched gas fluids (K+L) branched into the passage 81, and its flow rate becomes 50. In the low-pressure turbine process, on the other hand, most of the first oxygen-enriched gas fluid from the first condenser 12 in the passage 40 is introduced into the second rectification column 13, so that the flow rate becomes 58.

Therefore, the flow rate of the second product nitrogen gas (H) withdrawn from the second rectification column 13 differs in each process. That is, the flow rate is 19 in the intermediate-pressure turbine process, but it is increased to 22 in the low-pressure turbine process. Accordingly, with respect to the feed air having a flow rate of 100, the entire flow rate of the product nitrogen is 59 in the intermediate-pressure turbine process, but the flow rate is increased to 62 in the low-pressure turbine process.

In comparison of Table 2 and Table 3, a part of feed air is branched and is introduced into the air turbine in the air turbine process, so that the flow rate of the first product nitrogen gas (C) separated from the first rectification column 11 is reduced to 33 in the air turbine process, whereas it is 40 in the intermediate-pressure turbine process.

In the air turbine process, however, the first oxygen-enriched gas fluid in the passage 40, feed air from the expansion turbine in the air feeding passage 73 and the first oxygen-enriched liquefied fluid in the oxygen-enriched liquefied fluid feeding passage 74 are introduced into the second rectification column 13. Accordingly, the flow rate of the second product nitrogen gas (H) separated from the second rectification column 13 increases significantly. That is, the flow rate (H) in the intermediate-pressure turbine process is 19, whereas the flow rate in the air turbine process increases to 30. Therefore, though the entire flow rate of the product nitrogen in the intermediate-pressure turbine process is 59, the flow rate in the air turbine process increases to 63.

Table 4 shows the results of calculating the power consumption per production in each process.

TABLE 4 Inter- mediate- Low- pressure pressure Air turbine turbine turbine process process process First Feed flow rate A 100 100 100 rectification Nitrogen flow 40 40 33 column rate C Yield C/A 0.40 0.40 0.33 Second Material flow 50 58 67 rectification rate G column Nitrogen flow 19 22 30 rate H Yield H/G 0.38 0.38 0.45 Total Material flow 100 100 100 rate A Nitrogen flow 59 62 63 rate C + H = (X) Yield (X)/A 0.59 0.62 0.63 Required power Air compressor 100 100 100 Nitrogen 6 7 9 compressor Total (Y) 106 107 109 Power (Y)/(X) 1.80 1.73 1.73 Consumption Per Production

It is apparent from Table 4 that the power consumption per production of the low-pressure turbine process and air turbine process is about 4% less than that of the conventional intermediate-pressure turbine process. Although this is premised on the case where each rectification column is a sieve-tray type rectification column, a structured packing type rectification column, or a random packing type rectification column can be used for the individual rectification columns, and the use thereof provides nearly the same effects.

FIG. 4 is a systematic diagram showing one example of a nitrogen producing apparatus employing the conventional low-pressure process which uses one rectification column. In this nitrogen producing apparatus, a rectification column 91 is operated at a low pressure of about 0.5 MPa, the separated product nitrogen gas is compressed to a required pressure in a nitrogen compressor 92 and is supplied. The conventional process makes low power consumption per production possible, because a power consumption of feed air compressor (not shown) is reduced and a yield of the rectification column is improved. So, the process is used well. Because the descriptions of the individual parts of the apparatus and the flow rate of each gas-liquid in FIG. 4 are well-known conventionally, their descriptions will be omitted.

FIG. 5 shows relationships between product pressure and power consumption per production of the low-pressure process, low-pressure turbine process and air turbine process. It is apparent from FIG. 5 that a border exists at about 1.1 MPa of product pressure, the power consumption per production of the low-pressure turbine process and air turbine process are smaller at low pressure against the border, but the power consumption per production of the conventional low-pressure process is smaller at high pressure against the border.

In this case, as the product pressure is increased, the pressures of the first rectification column 11 and the second rectification column 13 increase, and the pressure of a waste gas also increases in the low-pressure turbine process and air turbine process. That is, though the ratio of the energy of the waste gas increases, the waste gas is merely depressurized and discharged, and the energy is not used effectively. Thus, as the product pressure is increased, the energy loss and the power consumption increase in the low-pressure turbine process and air turbine process, whereas the power consumption of the nitrogen compressor 92 merely increases in the low-pressure process.

It is understood from the results that a limit exists at 1.1 MPa of product pressure to obtain effects of the present invention in the low-pressure turbine process and air turbine process, that is, the pressure of the first product nitrogen gas which is separated from the top portion of the first rectification column 11 and is withdrawn from the first product recovery passage 35, has a limit to obtain the effects in both processes. Thus, the conventional low-pressure process is advantageous when product nitrogen is required to be sent to use point with higher pressure than the limit.

Table 5 shows the specifications of the expansion turbine in the low-pressure turbine process and air turbine process, which are compared with each other in a case where same quantity of product nitrogen is separated.

TABLE 5 Low-pressure Air turbine Process turbine process process Feed air 100 100 Expansion turbine Flow rate (=P) 31 15 Pressure (=Q) (MPa) 0.18 0.97 Volume flow rate (=P/Q) 172 16

In this example, the volume flow rates in both processes differ by about ten times (=172/16). This is because the pressure at the inlet of the expansion turbine is low so that the expansion ratio becomes smaller, and the turbine must process a relatively large quantity of fluid to obtain predetermined cold in the low-pressure expansion turbine process. The volume flow rate significantly affects the mechanical specifications (dimensions) of the expansion turbine.

The expansion turbine to be used in those processes is an ordinary general-purpose expansion turbine, so that when the volume flow rate is extremely low, there is a case where the use of a general-purpose turbine becomes difficult. When the volume flow rate is extremely high, there is also a case where the use of a general-purpose turbine becomes difficult or a plurality of expansion turbines should be provided.

When the low-pressure turbine process is employed in a relatively small-sized nitrogen producing apparatus, however, the process flow rate of the expansion turbine becomes greater for the size of the apparatus and does not become extremely low, thus it is possible to use an ordinary general-purpose expansion turbine. When the air turbine process is employed in a medium-sized nitrogen producing apparatus, on the other hand, the process flow rate of the expansion turbine can be made lower for the size of the apparatus, in which case it is also possible to use an ordinary general-purpose expansion turbine.

The ranges of the product pressure and product flow rate both advantageous in the low-pressure turbine process and air turbine process in consideration of this point are shown in FIG. 6. It is to be noted that because the process flow rate of the expansion turbine differs according to the conditions, such as the presence/absence of extraction of product liquefied nitrogen and the heat insulation state of the apparatus, it is desirable to determine selection of the low-pressure turbine process or the air turbine process in consideration of various conditions. Qualitatively, however, it can be said that in the case of a small-sized nitrogen producing apparatus whose product nitrogen flow rate is several thousand Nm³/h or lower, the low-pressure turbine process is advantageous, whereas in the case of a medium-sized nitrogen producing apparatus whose product nitrogen flow rate is several thousand Nm³/h or higher to about several ten thousand Nm³/h, the air turbine process is advantageous. 

1: A nitrogen producing process for collecting product nitrogen by performing cryogenic liquefaction separation of feed air, the nitrogen producing process comprising: a first separation step of performing low temperature distillation of compressed, purified and cooled feed air at a pressure of 0.8 MPa or higher and 1.1 MPa or lower to separate the feed air into a first nitrogen gas and a first oxygen-enriched liquefied fluid; a first indirect heat exchange step of performing an indirect heat exchange between the first nitrogen gas and the first oxygen-enriched liquefied fluid to condense the first nitrogen gas to obtain a first liquefied nitrogen and, at the same time, vaporize the first oxygen-enriched liquefied fluid to obtain a first oxygen-enriched gas fluid; a second separation step of performing low temperature distillation of the first oxygen-enriched gas fluid at a pressure of 0.4 MPa or higher and lower than the pressure in the first separation step to separate the first oxygen-enriched gas fluid into a second nitrogen gas and a second oxygen-enriched liquefied fluid; a second indirect heat exchange step of performing an indirect heat exchange between the second nitrogen gas and the second oxygen-enriched liquefied fluid to condense the second nitrogen gas to obtain a second liquefied nitrogen and, at the same time, vaporize the second oxygen-enriched liquefied fluid to obtain a second oxygen-enriched gas fluid; a cold generating step of generating cold needed for the operation by performing adiabatic expansion of the second oxygen-enriched gas fluid; a first product recovery step of discharging a part of the first nitrogen gas as a first product nitrogen gas after cold recovery; and a second product recovery step of discharging a part of the second nitrogen gas as a second product nitrogen gas after cold recovery. 2: A nitrogen producing process for collecting product nitrogen by performing cryogenic liquefaction separation of feed air, the nitrogen producing process comprising: a first separation step of performing low temperature distillation of compressed, purified and cooled feed air at a pressure of 0.6 MPa or higher and 1.1 MPa or lower to separate the feed air into a first nitrogen gas and a first oxygen-enriched liquefied fluid; a first indirect heat exchange step of performing an indirect heat exchange between the first nitrogen gas and the first oxygen-enriched liquefied fluid to condense the first nitrogen gas to obtain a first liquefied nitrogen and, at the same time, vaporize the first oxygen-enriched liquefied fluid to obtain a first oxygen-enriched gas fluid; a second separation step of performing low temperature distillation of the first oxygen-enriched gas fluid at a pressure of 0.3 MPa or higher and lower than the pressure in the first separation step to separate the first oxygen-enriched gas fluid into a second nitrogen gas and a second oxygen-enriched liquefied fluid; a second indirect heat exchange step of performing an indirect heat exchange between the second nitrogen gas and the second oxygen-enriched liquefied fluid to condense the second nitrogen gas to obtain a second liquefied nitrogen and, at the same time, vaporize the second oxygen-enriched liquefied fluid to obtain a second oxygen-enriched gas fluid; a cold generating step of generating cold needed for the operation by performing adiabatic expansion of a part of the feed air; an air feeding step of introducing the feed air undergone the cold generating step to an intermediate stage of the second separation step; a first product recovery step of discharging a part of the first nitrogen gas as a first product nitrogen gas after cold recovery; and a second product recovery step of discharging a part of the second nitrogen gas as a second product nitrogen gas after cold recovery. 3: The nitrogen producing process according to claim 1, comprising a step of compressing the second product nitrogen gas. 4: A nitrogen producing apparatus for collecting product nitrogen by performing cryogenic liquefaction separation of feed air, the nitrogen producing apparatus comprising: a first rectification column which performs low temperature distillation of compressed, purified and cooled feed air at a pressure of 0.8 MPa or higher and 1.1 MPa or lower to separate the feed air into a first nitrogen gas at the upper portion of the column and a first oxygen-enriched liquefied fluid at the bottom portion of the column; a first condenser which performs an indirect heat exchange between the first nitrogen gas and the first oxygen-enriched liquefied fluid to condense the first nitrogen gas to obtain a first liquefied nitrogen and, at the same time, vaporize the first oxygen-enriched liquefied fluid to obtain a first oxygen-enriched gas fluid; a second rectification column which performs low temperature distillation of the first oxygen-enriched gas fluid at a pressure of 0.4 MPa or higher and lower than the pressure of the first rectification column to rectify and separate the first oxygen-enriched gas fluid into a second nitrogen gas at the upper portion of the column and a second oxygen-enriched liquefied fluid at a bottom portion of the column; a second condenser which performs an indirect heat exchange between the second nitrogen gas and the second oxygen-enriched liquefied fluid to condense the second nitrogen gas to obtain a second liquefied nitrogen and, at the same time, vaporize the second oxygen-enriched liquefied fluid to obtain a second oxygen-enriched gas fluid; an expansion turbine which generates cold needed for the operation of the apparatus by performing adiabatic expansion of the second oxygen-enriched gas fluid; a first product recovery passage for discharging a part of the first nitrogen gas as a first product nitrogen gas after cold recovery; and a second product recovery passage for discharging a part of the second nitrogen gas as a second product nitrogen gas after cold recovery. 5: A nitrogen producing apparatus for collecting product nitrogen by performing cryogenic liquefaction separation of feed air, the nitrogen producing apparatus comprising: a first rectification column which performs low temperature distillation of compressed, purified and cooled feed air at a pressure of 0.6 MPa or higher and 1.1 MPa or lower to separate the feed air into a first nitrogen gas at the upper portion of the column and a first oxygen-enriched liquefied fluid at the bottom portion of the column; a first condenser which performs an indirect heat exchange between the first nitrogen gas and the first oxygen-enriched liquefied fluid to condense the first nitrogen gas to obtain a first liquefied nitrogen and, at the same time, vaporize the first oxygen-enriched liquefied fluid to obtain a first oxygen-enriched gas fluid; a second rectification column which performs low temperature distillation of the first oxygen-enriched gas fluid at a pressure of 0.3 MPa or higher and lower than the pressure of the first rectification column to rectify and separate the first oxygen-enriched gas fluid into a second nitrogen gas at the upper portion of the column and a second oxygen-enriched liquefied fluid at the bottom portion of the column; a second condenser which performs an indirect heat exchange between the second nitrogen gas and the second oxygen-enriched liquefied fluid to condense the second nitrogen gas to obtain a second liquefied nitrogen and, at the same time, vaporize the second oxygen-enriched liquefied fluid to obtain a second oxygen-enriched gas fluid; an expansion turbine which generates cold needed for the operation of the apparatus by performing adiabatic expansion of a part of the feed air; an air feeding passage for introducing the feed air passed through the expansion turbine to an intermediate stage of the second rectification column; a first product recovery passage for discharging a part of the first nitrogen gas as a first product nitrogen gas after cold recovery; and a second product recovery passage for discharging a part of the second nitrogen gas as a second product nitrogen gas after cold recovery. 6: The nitrogen producing apparatus according to claim 4, comprising a nitrogen compressor which compresses the second product nitrogen gas. 7: The nitrogen producing apparatus according to claim 4, wherein the second rectification column has a liquefied nitrogen feeding passage for introducing liquefied nitrogen from outside the apparatus. 8: The nitrogen producing process according to claim 2, comprising a step of compressing the second product nitrogen gas. 9: The nitrogen producing apparatus according to claim 5, comprising a nitrogen compressor which compresses the second product nitrogen gas. 10: The nitrogen producing apparatus according to claim 5, wherein the second rectification column has a liquefied nitrogen feeding passage for introducing liquefied nitrogen from outside the apparatus. 